In the real world
objects of a scene do not only receive light from the main light source.

In fact each object reflects a part of
the light on the nearest objects. This effect is usually more or
less rendered in the using of an ambient light.

However, this ambient light is far
from being satisfactory, because it doesn't take the colour into account.
You can notice on the picture here aside the importance of the red and
green reflects from the walls onto the objects in the centre. To obtain
this result there is a global computation method for the illumination of
the scene that we call : radiosity. This is a great speciality of the Cornell
university which experimental model, the famous cornell's box is now the
reference in the domain.

However radiosity is a hard stuff to use
either by means or by the time of computation it needs.

Even if Blender offers a radiosity engine,
it is preferable to try simulating it's effects. That's what is explained
on the next page.